with C:\Program Files (x86)\MiKTeX 2.9\scripts\tex4ht\htlatex.bat mwe.tex (after it was succesfully compiled with pdflatex) produces an ANSI rather than an UTF-8 encoded HTML file. That is, mwe.html once opened shows

chars with ΰccιθnts greek chars ασδφγη

and I can display chars correctly only if I manually change browser encoding to ISO-8859-1.

I also tried a bunch of options for htlatex (the compilation output of htlatex mwe "xhtml, charset=utf-8" " -cunihtf -utf8" is available here), but none solved the problem, any clue?

FOLLOW UP

Since the solution given in answer works fine with TeXLive under Ubuntu, I guess the issue with MikTeX under Windows is due to MikTex.

Did it, same wrong result and same exit code 1 for same reason.
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mmjJun 13 '13 at 16:10

@mmj: Michal's command line works fine for me on miktex 2.9. Can you show the complete command line output?
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Ulrike FischerJun 13 '13 at 16:17

@mmj as @Ulrike said, please show us complete log file, it may be some problem with your tex4ht instalation
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michal.h21Jun 13 '13 at 16:26

1

Thanks, I added in the question a link to the output. I found out that the path problem does not concern tex4ht (it shows up even when I open a command line window!). I also discovered that, with your options, actually no new html file is produced, I was looking at the previously generated html file, due to --- error --- Illegal storage address, which you will find at the end of output. Such error does not show up compiling without options.
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mmjJun 13 '13 at 23:25

I could not make your solution work under MikTeX, so I switched to TeXLive under Ubuntu and it worked. In general, switching from MikTeX under Windows to TeXLive under Ubuntu let me solve not only this one, but other severe issues too.
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mmjDec 2 '13 at 9:03